1. Field of the Invention
The present invention relates to a laser projection display apparatus, and more particularly, to a laser projection display apparatus designed to determine the point in time of starting projecting an image signal by acquiring information on the rotation position of a rotating polygonal mirror.
2. Description of the Related Art
Currently, a cathode ray tube (CRT) and a liquid crystal display (LCD) are used as an image display device. However, realization of a large screen of the CRT or LCD is difficult, and the resolution of images is low.
A laser projection display apparatus obtains colors close to natural colors while satisfying a demand for an enlarged and high resolution display device. Also, the laser projection display apparatus is being applied in various fields such as electronic billboards, laser shows, and large-sized television image displays because of its high luminance property.
The laser projection display apparatus vertically projects laser beams, emitted from an optical source, onto a screen by means of a galvanometer, and horizontally projects the laser beams by means of a rotating polygonal mirror, thereby displaying images. As for television images, laser beams are vertically projected in a saw tooth wave of 60 Hz, and horizontally projected at 15.72 KHz. At this time, the galvanometer sufficiently can make a mechanical response, so that vertical projection of laser beams can be smoothly performed. However, horizontal projection is liable to be irregular, causing picture jitter. One reason for such irregular horizontal projection is that it is difficult to constantly keep the rotation speed of a rotating polygonal mirror. In particular, a rotating polygonal mirror rotating 656 times per second must keep a rotation speed error within about .+-.0.1% to suppress horizontal picture jitter. However, this rotation speed control is very difficult to realize in practice. Another reason for the irregular horizontal projection is that horizontal projection does not always start on the same perpendicular line since not all 24 reflection planes of a rotating polygonal mirror can have the equal state of machining.